In general, a railroad track comprises a pair of rails which are secured to ties or sleepers extending transversely of these rails anchored somewhat yieldably in a bed of ballast generally formed by gravel and disposed along the railroad right-of-way.
This ballast bed is generally provided upon a graded and compacted surface or even upon a concrete platform.
With increasing desire for quiet travel and because of the limited yieldability of the gravel ballast, efforts have been made to increase the resiliency of the bed by providing mats of elastic material between the ballast and the support surface. These cushioning mats can be comprised of elastomeric material and especially from reclaimed rubber and have, in the past, been constituted by one or more layers of foam material or from rubber strips ensheathed in reclaimed rubber granules which are adhesively secured to the core.
Such rubber cushioning layers have greatly improved the resiliency of the bedding for the track and have served as effective noise-damping levels in tunnels or elsewhere along the right-of-way where noise problems are pronounced.
However, these systems do have the disadvantage that a relatively sharp-edged gravel ballast penetrates into the mat in a fairly short time, damaging the latter and/or markedly reducing its elasticity.
As the mats tend to be penetrated by the ballast, they become more and more rigid and hence less and less able to serve as vibration-damping or sound-damping members.
Another important disadvantage of these earlier mats is that they tend to shift laterally or longitudinally in the direction of horizontal forces which are generated as the ballast is pressed downwardly against the mats.
As a result of this shifting, on one side the ballast may be directly in contact with the support surface while on the opposite side the ballast may be supported by the cushioning layers.
The locations at which the ballast is no longer supported by the shifted cushioning layers, the sound-damping effect is markedly reduced and the noise generated may become excessive.
Another disadvantage of the earlier bedding mats is that, especially when the ballast is constituted by large rocks and perimetal-shaped members, there is a tendency for the tremendous pressures applied in the vertical direction to drive these ballast-forming members through the mats and form bridges of the ballast material which create sound-transmitting paths avoiding the sound-damping effects described above.